1. Field of the Invention
The present invention relates to a method for implementing and operating a read/write unit in a system that includes multiple wirelessly readable transponders, and to such a read/write unit implementing the said method.
2. Description of the Related Art
By means of Radio Frequency Identification (RFID) technology, RFID labels, which are commonly designated transponders or “tags”, are registered and read wirelessly by read/write units (“readers”). The transponders are normally activated by an electromagnetic field, which is generated by a read/write unit by emitting a carrier wave with sufficient transmitted power, and are typically also supplied with energy (passive transponders). Transponders are also known in which the energy supply of the transponder is provided by a dedicated power supply, such as batteries (i.e., active transponders).
RFID technology is generally used where objects, such as workpieces or dispatch items, are intended to be registered and identified unambiguously via radio in the surroundings or proximity of a read/write unit. For this purpose, a read/write unit is able to store information, which comprises at least one unique identification number, on the transponder via radio and to re-read the stored information. In a working environment, e.g., in industrial automation systems, the read/write unit is often connected to a control unit (e.g., an industrial controller “PLC” or personal computer).
In particular, when RFID technology is used in industrial production plants, there is often a need to identify unambiguously exactly that object and therefore exactly that transponder which is located in the immediate vicinity of the read/write unit, in order to then perform appropriate fabrication steps on the object. On the other hand, in such industrial automation surroundings, remote-field RFID systems, as they are known, are frequently used, which systems typically permit the detection of transponders located within a range or area of several meters around a read/write unit. In densely packed surroundings, when a plurality of objects with their associated transponders are located close to one another, it frequently therefore occurs that an RFID read/write unit simultaneously detects a plurality of transponders in its acquisition range. Consequently, in addition to the transponder located closest to the read/write unit and the antenna of the read/write unit, still additional transponders are detected. In this case, an unambiguous assignment of the object to be processed or to be handled at this time is no longer possible, so that errors can occur in downstream processes.
In order to solve this problem, it is known to reduce the transmitted power of the read/write unit such that, on account of the lower range associated with the reduction in transmitted power, only a maximum of one single transponder is recognized. In such cases, it has proven to be disadvantageous that the transmitted power falls to such an extent or has to be reduced to such an extent that, in many cases, the desired (“correct”) transponder can no longer be reliably detected and recognized. Particularly where, because of the process, there is a relatively long time interval between reading and writing data to and from the same transponder, it is not possible to ensure that the “correct” transponder is recognized/detected clearly at both times and that unambiguous communication with this particular transponder is possible in each case.
A similar solution, which is often associated with a reduction in the transmitted power as described, is to configure the radio range with insulating plates and reflectors such that the radio range is restricted to the appropriate workstation, and thus the undesired detection of objects further away and the transponders thereof is prevented. However, this is often difficult to effect in the specific instance and commonly leads to higher costs.
Another strategy for solving the above-described problem resides in permitting simultaneous detection and simultaneous operation of a communications relationship with a plurality of transponders. Here, by means of a control device with which the read/write unit is linked, appropriate evaluation logic is implemented, by which the multiplicity of simultaneously existing communications relationships are managed and by which it is ensured that the operations assigned to the “correct” (i.e., normally the closest) transponder are performed at an appropriate processing station or the like. However, the disadvantage of this is that the corresponding processing logic for concurrently managing a plurality of communications relationships must be integrated into the logic of a process control system and, these systems often being proprietary in nature, such processing logic must be implemented in the corresponding control device on a case-by-case basis matched to the specific situation.